Randomly accessible visual information recording medium and recording method, and reproducing device and reproducing method

ABSTRACT

Access point pictures designated as randomly accessible positions are I pictures or P pictures. Information indicating the decoding sequence (I 1,  P 1,  B 1,  B 2,  B 3,  B 4,  P 2,  . . . ) of pictures functioning as access points and attribute information (picture_type) indicating whether a picture functions as an access point or is necessary for decoding of the access point following a given access point are recorded on the video information recording medium. Random access is possible even if the GOP interval is lengthened.

This application is a Continuation of co-pending application Ser. No.10/569,603, filed on Feb. 24, 2006, and for which priority is claimedunder 35 U.S.C. §120. Application Ser. No. 10/569,603 is the nationalphase of PCT International Application No. PCT/JP05/11342 filed on Jun.21, 2005 under 35 U.S.C. §371. This application claims priority ofApplication Nos. 2004-195476, 2004-205250, 2004-214080, 2004-229683, and2004-238482 all filed in Japan on Jul. 1, 2004, Jul. 12, 2004, Jul. 22,2004, Aug. 5, 2004, and Aug. 18, 2004, respectively, under 35 U.S.C.§119; the entire contents of all are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a randomly accessible video informationrecording medium, a recording device for recording video data on a videoinformation recording medium, the recording method, a reproducing devicefor reproducing video data from a randomly accessible video informationrecording medium, and the reproducing method.

BACKGROUND ART

A proposed system for writing encoded image data in packets formattedaccording to the MPEG system on a storage medium to enable trickreproduction of image data by a simple and efficient method withoutincreasing the capacity of the storage medium is to set up an I pictureindex of packets in which at least part of the I picture data is storedand, during trick reproduction, to read only the packets on which the Ipicture index is set (for example, Patent Document No. 1).

-   Patent Document No. 1: Japanese Patent Application Publication No.    H9-98430 (pp. 4-10, FIGS. 1-15)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

In recent years, MPEG4-AVC (H.264) and other new encoding methods bywhich even low-bit-rate encoding can provide adequate image quality havecome into use. To obtain high image quality with a low bit rate, it isnecessary to minimize the number of intra-coded (I) pictures, whichrequire relatively much encoded data.

Because the first frame in a group of pictures (GOP) must be an Ipicture, reducing the number of I pictures is equivalent to increasingthe GOP length. For example, a 1-seg broadcast, which is a televisionbroadcast for mobile terminals, allows up to a five-second GOP length.If the GOP length is increased in this manner, the number of positionsthat can be designated as access points for random access is greatlyreduced, because random access must start from an I picture, which canbe decoded without reference to other frames. In a time search etc. of avideo image recorded by a user, accordingly, because the only positionsdesignated as accessible points are the I pictures at which GOPs begin,although the user may want to reproduce the image starting from acertain point in time, access to precisely that point of time may bedifficult; to the user's inconvenience, it may only be possible toreproduce the image starting from a time offset from the desired pointin time. Another problem is that if the GOP length is set to fiveseconds, for example, in random access to a point within that interval,in the worst case more than two seconds passes before the desiredpicture is reproduced.

The present invention addresses the above problems, with the object ofproviding a video information recording medium, a video informationrecording apparatus and recording method, and a reproducing apparatusand reproducing method with which random access is possible even if theGOP length is increased.

Means of Solution of the Problems

This invention is a video information recording medium on which arerecorded video data organized into video units including intra-coded (I)pictures coded intra-frame, predictive coded (P) pictures each includinga group of blocks predicted from one frame, and bidirectionallypredictive coded (B) pictures each including a group of blocks predictedfrom two frames, the video information recording medium being randomlyaccessible and having disposed thereon information indicating a decodingsequence of pictures designated as randomly accessible positions andthus functioning as access points, and attribute information indicatingwhether a picture functions as an access point or is necessary fordecoding of the access point following a given access point.

EFFECT OF THE INVENTION

According to this invention, video reproduction can be carried outsmoothly from the point in time desired by the user even if the GOPlength is increased, and random access is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) and FIG. 1(b) are diagrams illustrating the structure of a GOPaccording to the first embodiment.

FIG. 2 shows an example of the GOP structure when two P picture accesspoints are set in one GOP.

FIG. 3 illustrates the structure of index information for accessing a Ppicture used as an access point.

FIG. 4 is a block diagram illustrating the structure of a reproducingdevice according to the first, a fourth, and a fifth embodiment.

FIG. 5 illustrates the relationship between a video file on an opticaldisc and the management data in Entry_map( ) in the first embodiment.

FIG. 6 illustrates the structure of index information for accessing a Ppicture used as an access point in a second embodiment.

FIG. 7 illustrates the relationship between picture references andframe_num in a GOP in the second embodiment.

FIG. 8 illustrates the structure of a GOP according to a thirdembodiment.

FIG. 9 is a block diagram showing the structure of a reproducing deviceaccording to the third embodiment.

FIG. 10 illustrates the structure of GOP_access_info, which describesaccess information in each GOP in the third embodiment.

FIG. 11 illustrates another example of the structure of theGOP_access_info describing access information in each GOP in the thirdembodiment.

FIG. 12 illustrates the structure of index information for accessing a Ppicture used as an access point in the third embodiment.

FIG. 13 illustrates the relationship between the picture sequencearranged in presentation order and the sequence rearranged at the timeof recording for random access in a fourth embodiment.

FIG. 14 illustrates the structure of GOP_structure( ) in the fourthembodiment.

FIG. 15 illustrates the relationship between Entry_map( ) and thepicture sequence in the fourth embodiment.

FIG. 16 illustrates the relationship between the picture sequencearranged in presentation order and the sequence rearranged at the timeof recording for random access and fast-forward reproduction in a fifthembodiment.

FIG. 17 illustrates the structure of index information supporting randomaccess and fast-forward reproduction in the fifth embodiment.

EXPLANATION OF REFERENCE CHARACTERS

AP, AP1, AP2 access points, 1 navigation data, 2 video data, 3 videofile, 4 GOP(k) 100, 120 reproducing device, 101 user interface, 102 CPU,103 drive, 104 drive controller, 105 work memory, 106 system bus, 107decoder, 108 buffer memory, 109 display device, 110 picture selector

BEST MODE OF PRACTICING THE INVENTION

Embodiments of the invention will now be described with reference to theattached drawings.

The embodiments illustrate mainly the case in which the videoinformation recording medium is an optical disc, but it may be anotherrecording medium such as a hard disc or semiconductor memory.

These embodiments will be described for the case in which a 30 framesper second picture is compressed to digital video data by the MPEG4-AVC(Advanced Video Codec) encoding system. Each frame of digital video dataconsists of one of three types of coded picture: an I picture, a Ppicture, or a B picture. An I picture is an intra-coded picture codedwithin one frame. A P picture is a predictive coded picture; one frameis divided into a group of blocks, each block being predicted from oneother frame; that is, a P picture is a group of blocks predicted fromone frame. A B picture is also a predictive coded picture in which oneframe is divided into a group of blocks, but each block is predictedwith reference to two other frames; that is, a B picture is a group ofblocks predicted from two frames. A group of pictures (GOP) isstructured as a unit comprising at least one I picture followed by oneor more P pictures and one or more B pictures, and the video dataconsist of a plurality of such GOPs. The MPEG4-AVC standard provides nodefinition of GOPs, but the term GOP will be used herein to denote thisconcept as applied to MPEG4-AVC, and the following description willassume that the length of a GOP is 1.0 second.

First Embodiment

FIG. 1(a) and FIG. 1(b) are diagrams illustrating the structure of aGOP. In these figures, I represents an I picture, P represents a Ppicture, and B represents a B picture. The leftmost picture I1 is theoldest picture, and subsequent pictures are arranged temporally fromleft to right in their presentation sequence. In the figures, arrowsrepresent predictive and referencing encoding relationships among thepictures. The numbers after the letters I, P, and B are pictureidentification numbers, given in temporal sequence.

FIG. 1(a) is an example of the MPEG4-AVC coded GOP structure. How thepictures are decoded will be described with reference to FIG. 1(a). Thepictures are displayed in the order shown in the figure, but arerecorded on a recording or other medium in the decoding sequence, suchas in the order of I1, P1, B1, B2, B3, B4, P2 . . . in FIG. 1(a), forexample.

In FIG. 1(a), I1 is an independently decodable picture, and because itis the first picture, it is decoded first. The decoded I picture isnecessary for later decoding, so it is temporarily stored in a framememory in the decoder. Next, P1 is decoded, and the decoded P picture isalso temporarily stored in the frame memory. Then B1 is decoded withreference to I1 and P1, which are already decoded, and is temporarilystored in the frame memory. I1 is deleted from the frame memory at thistime. Next B2 is decoded with reference to B1 and P1, and is temporarilystored in the frame memory. B3 is decoded with reference to P1 and B2and is temporarily stored in the frame memory, but is then deleted fromthe frame memory. B4 is decoded with reference to P1 and B3 and istemporarily stored in the frame memory, and all the pictures in theframe memory except B4 are discarded. Next P2, which is predicted fromB4, is decoded and temporarily stored in the frame memory. Decodingproceeds in a similar manner.

As described above, to decode one picture, a temporally precedingpicture and/or a following picture is necessary, which poses asignificant problem for random access. Consider P2: in order to decodeP2, B4 is necessary, and in order to decode B4, B3, and P1 arenecessary. In order to decode B3, P1, and B2 are necessary, and in orderto decode B2, B1, and P1 are necessary. In order to decode B1, I1, andP1 are necessary. Therefore, if one wants to start reproducing from P2,one finds that P2 can be decoded only after decoding all of pictures I1,B1, B2, P1, B3, and B4. (In MPEG2, which, unlike MPEG4-AVC, does notpermit the pictures used for reference and prediction to be selectedarbitrarily, P2 could be decoded and reproduced by decoding I1 and P1.)This means that a picture encoded by MPEG4-AVC lacks randomaccessibility. In low-bit-rate encoding, the GOP length tends to be sethigh in order to improve the coding efficiency. If, for example, thelength of a GOP is about five seconds, then in random access to apicture in the interior of the GOP, in the worst case, more than twoseconds passes before the desired picture is reproduced.

Therefore, in an encoding scheme such as MPEG4-AVC which allowsarbitrary selection of the pictures used for reference and prediction,many pictures have to be decoded in order to access a picture in theinterior of a GOP, and time is necessary before reproduction begins. Asa result, problems arise in starting reproduction from the point desiredby the user: it may take time for reproduction to begin, or reproductionmay start from a point different than the desired point.

To solve these problems, it is necessary to add some constraints onencoding. Adding strong constraints, however, would weaken theadvantages of MPEG4-AVC, so the constraints should be minimized. FIG.1(b) shows the structure of a GOP in which a picture to be used as anaccess point (AP) has been coded with certain constraints. The picturesare arranged in their presentation sequence from left to right. For thisGOP, the following three constraints are added as conditions for codingpictures following the access point AP:

-   1. A picture used as an access point AP must be an I or P picture;-   2. A P picture used as an access point AP must always be predicted    from the initial I picture of the GOP; and-   3. Pictures following a P picture used as an access point AP must    not be predicted or referenced from pictures preceding the access    point AP, other than the initial I picture of the GOP.

An access point AP is a position (point) accessible in random accessreproduction, in which reproduction starts from an arbitrary point inthe video information, e.g., a point desired by the user. For aread-only optical disc, these points are designated at the time ofauthoring of the disc, while for a recordable or rewritable opticaldisc, the points are designated by the recording device when it recordsvideo data on the disc.

In FIG. 1(b), in order to start reproduction from P4, which is an accesspoint AP, all that is necessary is to decode I1 at the start of the GOP,and then decode P4. Pictures following P4 do not use pictures precedingP4 for reference or prediction, so subsequent pictures can be reproducedcontinuously by decoding them in the usual sequence.

When encoding is carried out with the above constraints, in order tostart reproduction from P4, which is an access point AP, it is onlynecessary to read and decode the data of I1 at the start of the GOP; itis not necessary to read or decode other pictures preceding P4. Let itbe assumed that the disc read-out rate is 10 Mbps, the coding rate is 10Mbps, the coding ratio of I pictures to B pictures to P pictures is10:6:1, and the seek time is 100 msec. In this case, the total data sizeof I1 and P4 will be about 1.3 Mbits under the above conditions, and thetime necessary to read out all this data will be 130 msec. After readingI1, an additional 100 msec is needed to access P4, so about 230 msecwill be necessary in all.

The above discussion deals with a case in which there is a single Ppicture access point AP in the GOP, but it is also possible to set aplurality of P pictures as access points in the GOP. FIG. 2 shows anexemplary GOP structure with two P picture access points AP1 and AP2 inone GOP. P4, which is the first P picture access point AP1, is the sameas in the example above with a single P picture access point AP, so anexplanation will be omitted. The second access point AP2 is P8. P4 isencoded by prediction from I1, but P8, the second access point AP2, istemporally more remote from I1. In general, the more temporally remotethe picture to be predicted is, the less accurate the predictionbecomes, resulting in degradation of picture quality. Therefore, it isbetter for P pictures at and after the second access point AP2 to bepredicted from the P picture at the immediately preceding access point(e.g., AP1) than from the I picture. P8, positioned at the second accesspoint AP2, is encoded by prediction from P4 at the first access pointAP1. It is not always necessary to predict from the immediatelypreceding access point, however, because prediction from I1 maysometimes allow more efficient encoding.

Therefore, the encoding constraints for the second and subsequent Ppicture access points are as follows:

-   1. A picture used as an access point must be an I or P picture;-   2. A P picture used as an access point must be predicted from the    initial I picture of the GOP, or another P picture used as an access    point; and-   3. Pictures following a P picture used as an access point must not    be predicted or referenced from pictures preceding the access point,    other than the initial I picture of the GOP or a P picture used as    an access point.

When the user wants to start reproduction from P8, which is used as thesecond access point AP2, I1 at the start of the GOP is reproduced first.Next, P4, which is the first access point AP1, is read and decoded; thenP8 is read and decoded. Subsequent reproduction can be performed in theusual manner. Differing from the case of a single access point AP, whenthere are a plurality of access points, it is necessary to reproduce theinitial I picture of the GOP and the P pictures at access pointspreceding the intended access point, so the farther back in the GOP theaccess point is, the more time is needed before reproduction begins.Nevertheless, the pictures can be reproduced considerably more quicklythan if they had been encoded without these constraints.

Under the above encoding conditions, the following index information isadded to the picture data to facilitate reproduction from an accesspoint in the interior of a GOP. FIG. 3 is shows the structure of theindex information for accessing a P picture used as an access point. InFIG. 3, Entry_map( ) is a data area in which information necessary foraccess points is stored; Entry_map( ) constitutes part of the navigationdata. Navigation data refers in general to control information andmanagement information for controlling reproduction of contents such asvideo files on a recoding medium, and includes the access point indexinformation.

number_of_IAP in FIG. 3 gives the total number of I pictures in a motionvideo file. A motion video file consists of a plurality of GOPs. Theinitial I picture of a GOP is not necessarily used as an access point,and accordingly the value indicated in number_of_IAP need not be thesame as the number of the GOPs, but it never exceeds the total number ofthe GOPs.

The for loop statement following number_of_IAP in FIG. 3 is a looprepeated this (number_of_IAP) number of times. The following accesspoint information is coded therein, for the interval from one I pictureaccess point to the next I picture access point. This may be regarded asper GOP access point information, because in a GOP on the order ofseveral seconds long, the I picture at the beginning of the GOP isalways designated as an access point.

The I_PTS_AP[IAP_id] in FIG. 3 gives the presentation time of an Ipicture, which is display time information indicating the reproductiontiming at the start of the access point. The presentation time may beeither the PTS given for each picture in MPEG2, or the relative timefrom the initial picture in the video file. The bracketed [IAP_id] isthe number of a particular I picture access point among thenumber_of_IAP I picture access points. I_PTS_AP[IAP_id], for example,gives I_PTS_AP information for the I picture access point designated by[IAP_id]. The meaning of this notation is the same below, so subsequentexplanations will be omitted.

The next item, I_SCN_AP, is information giving the position in the videofile or on the disc of the initial I picture access point. In thisembodiment, the sector offset of the I picture relative to the start ofthe video file is given. It will be appreciated that a byte offset orthe like can be used instead of a sector offset: any information may beused that can identify the position of the I picture relative to thestart of the video file, or the absolute position of the I picture onthe disc. Size_of_IAP is information giving the data size of the initialI picture access point. In this example, the sector offset of the sectorcontaining the last byte of the I picture relative to the start of theGOP including the I picture is given. From these three items of accesspoint information, the starting point, presentation time, and data sizeof the initial I picture access point can be identified.

In number_of_PAP in FIG. 3, the total number of P picture access pointsfrom one I picture access point to next I picture access point, that is,the total number of P picture access points until the start of the nextGOP, is given. The following for loop statement is a loop repeated this(number_of_PAP) number of times.

P_PTS_AP[IAP_id] [PAP_id] is similar to I_PTS_AP above, giving thepresentation time, indicating the display timing, of a P picture used asan access point. In this embodiment, the presentation time may be thePTS given for each picture in MPEG2, or the relative time from the startof the video file. In general, the less data other than video data thereis, the better. For a clock frequency of 90 kHz, an absolute PTSrequires 33 bits of data, but if the maximum GOP length is five seconds,the amount of data required for a relative PTS is reduced to 19 bits, sothe value of P_PTS_AP[IAP_id] [PAP_id] is preferably the relative timefrom the start of the GOP. In this embodiment, the presentation time isrelative to the start of the video file. The bracketed [IAP_id] [PAP_id]gives information that corresponds to the number of a particular Ppicture access point among the (number_of_PAP) P picture access pointsfrom the I picture identified by IAP_id to the next I picture accesspoint. For example, P_PTS_AP[IAP_id] [PAP_id] designates the P_PTS_APinformation of the P picture access point identified by [PAP_id] amongthe P picture access points from the single I picture access pointidentified by [IAP_id] to the next I picture access point. The meaningof this notation is the same below, so subsequent explanations will beomitted.

The next item, P_SCN_AP, is similar to the above I_SCN_AP, givinginformation about the position at which a P picture access point startsin a GOP, or in the video file. The sector offset relative to the startof the I picture access point is given here. The Size_of_PAP informationis similar to Size_of_IAP, giving the data size of the P picture accesspoint. The sector offset of the sector including the last byte of the Ppicture used as an access point, relative to the start of the P picture,is given. From the above three items of information, the video imagestarting position, presentation time, and data size of a P picture usedas an access point can be identified.

Structuring the access point index information as described aboveenables positional information, temporal information, and the picturesize of each access point to be identified. Next, a procedure for usingthe above index information to start reproduction from an access pointwill be described. FIG. 4 shows the structure of a reproducing device100, and FIG. 5 shows the relationship between a picture file on anoptical disc and the management data in Entry_map( ).

First, an example of the ordinary reproduction sequence will bedescribed. When a reproduce instruction from the user is input throughthe user interface (I/F) 101 to the CPU 102, a command to readnavigation data 1 is output to the drive controller 104, which controlsthe drive 103 that reads data from an optical disc. The read-outnavigation data 1 are transferred via the drive controller 104 to a workmemory 105. The drive controller 104 and the work memory 105 areconnected to the CPU 102 via a system bus 106 comprising an address busand a data bus, which is used to transfer commands from the CPU 102 andtransfer data between blocks. The CPU 102 extracts managementinformation regarding the program designated for reproduction by theuser from the navigation data 1 deployed in the work memory 105. Basedon the extracted management information, the CPU 102 instructs the drivecontroller 104 to read the data of the video file 3 required forreproduction from the video data 2, and the drive 103 reads out thedesired data. The read-out data are temporarily stored, via the systembus 106, in a buffer memory 108 of the decoder 107, which decodes thecoded data. The CPU 102 controls this process to prevent the buffermemory 108 from being exhausted or overflowing, in order to achievereproduction without interruption in picture or sound. The data storedtemporarily in the buffer memory 108 are decoded by the decoder 107 intoa video signal, which is output to a display device 109 such as a TVmonitor.

Next, the flow of processing in a time search, in which the userdesignates a time, will be described. When the user interface 101outputs a search instruction having a time specified by the user, theCPU 102 refers to the Entry_map( ) of the navigation data 1 stored inthe work memory 105. Among the I_PTS_AP in Entry_map( ), let I_PTS_AP(k)be the time information of the I picture access point closest to thetime specified by the user. Among the P_PTS_AP, let P_PTS_AP(j) be thetime information of the closest P picture access point (where j is aninteger equal to or greater than one, indicating the j-th P pictureaccess point counted from the I picture). The above I picture will beassumed to belong to GOP(k) 4.

The starting address of the video file 3 currently being reproduced canbe recognized from the file system in the storage medium. Therefore, theabsolute address of the initial I picture in the GOP(k) 4 containing theintended access point will be the starting address of the video file 3plus the I picture position information I_SCN_AP(k). The CPU 102instructs the drive controller 104 to read data from this absoluteaddress. The amount of data read is equivalent to the number of sectorsgiven in Size_of_IAP, which is the data size of the I picture. The Ipicture data read according to the position information and data sizeare temporarily loaded from the drive controller 104 into the buffermemory 108.

When the reading of the I picture is completed, the CPU 102 instructsthe drive controller 104 to read data from an address determined byadding the following three addresses: the starting address of the videofile, the I picture position information I_SCN_AP(k), and the positioninformation of the next P picture access point (P_SCN_AP(1), not shown).The amount of data read is equivalent to the number of the sectors givenin Size_of_PAP(1). The data of the P picture thus read according to theposition information and data size is temporarily loaded from the drivecontroller 104 into the buffer memory 108.

In order to reproduce P picture j, it suffices to read the I picture inGOP(k) 4 and the P pictures at the following access points AP(1) toAP(j), so the above process is repeated j times. The intended accesspoint P_PTS_AP(j) is thereby reached.

The CPU 102 calculates timings to output video from the intended accesspoint within the shortest period, transfers data stored in the buffermemory 108 to the decoder 107, and starts decoding. Reproduction fromthe user-specified time is carried out in the above manner. The abovedescription is for a time search process in which the user specifies thetime at which to start reproduction, but it will be appreciated that theuser can specify the picture or address at which reproduction is tostart; the specified reproduction position in the present inventionincludes positions specified by time, address, or picture etc.

As described above, using I pictures and P pictures as access pointsdesignated as randomly accessible positions enables access points to bedesignated at reasonable intervals, without reducing the number ofaccess points, even in low-bit-rate coding systems such as MPEG4-AVC.

When a picture used as an access point is a P picture, coding efficiencycan be maintained by coding the picture by prediction either from atemporally preceding P picture or from the initial I picture in the GOP.

Furthermore, reproduction following the access point can proceedsmoothly because pictures temporally following an I picture or a Ppicture used as an access point are not coded by prediction frompictures temporally preceding the I picture or the P picture used as theaccess point, other than the initial I picture in the GOP and otherpictures used as access points.

Storing information on the optical disc or other video informationrecording medium giving the presentation time, position, and data sizeof pictures used as access points enables quick reproduction from accesspoints in the interiors of GOPs.

According to the present embodiment, compression efficiency can beimproved by increasing the GOP length without compromising randomaccessibility.

Second Embodiment

In the first embodiment, P_PTS_AP[IAP_id] [PAP_id], which indicatesrelative time from the start of the GOP, was used as information givingthe presentation time of the access points in the GOP, but a method ofobtaining time information by using information with a smaller data sizewill now be described. FIG. 6 shows a new structure of Entry_map( )which differs from the Entry_map( ) shown in FIG. 3 in thatframe_num[IAP_id] [PAP_id] is used instead of P_PTS_AP[IAP_id] [PAP_id].frame_num is a parameter given in the slice header in an MPEG4-AVCstream. frame_num is incremented every time a reference picture isdecoded, and takes on 4-bit to 16-bit values. FIG. 7 shows therelationship between frame_num and the picture references in a GOP. Asshown in FIG. 7, the value of frame_num is incremented by one every timea reference picture is decoded. The display order of an access point iscalculated by the following equation.Display order [IAP_id] [PAP_id]=(frame_num[IAP_id] [PAP_id]−frame_num[IAP_id])*M

M in this equation is a value indicating the interval between Ppictures: in FIG. 7, M=3. When calculated according to the aboveequation, the indicated access point works out to be fifteenth in thedisplay order. On the assumption that the PTS clock frequency is 90 kHzand the frame frequency is 24 Hz, the PTS difference between frames is90000/24=3750, so the PTS value relative to the initial picture of theGOP is calculated by the following equation.P _(—) PTS _(—) AP[IAP_id] [PAP_id]=Display_order[IAP_id] [PAP_id]*3750

To make calculations such as in the two equations above possible, thefollowing encoding constraints must be satisfied.

-   1. No B picture is used as a reference picture.-   2. No reference is made that causes a reordering of reference    pictures.-   3. The number of B pictures between two reference pictures in the    GOP is fixed.    When the above conditions are satisfied, the presentation time of an    access point can be calculated easily from the value of frame_num.    The data size of frame_num is 16 bits at maximum, but it is not    necessary for all the frame_num bits to be used. For example, if the    maximum GOP length is five seconds, the frame frequency is 30 Hz,    and M=3, then the maximum number of frames in a GOP is 150, of which    50 are P pictures. Therefore, the relative frame offset from the I    picture can be calculated using the least significant six bits in    frame_num.

As described above, if predetermined constraints are satisfied duringencoding, the presentation time of a P picture used as an access pointcan be calculated easily from frame_num and the amount of data otherthan video data can be reduced, compared with use of PTS. Although thepresentation time of an access point is calculated from frame_num inthis embodiment, the presentation time of an access point can besimilarly calculated from any information that indicates the decodingorder.

Third Embodiment

In the first embodiment, a method of accessing an access point at a Ppicture in the interior of a GOP was described. In the presentembodiment, a reproducing method with high image quality as well as highaccess speed, obtained by control of the encoding system, will bedescribed.

In the first embodiment, the picture used for prediction of a P pictureat an access point was a P picture at another access point or theinitial I picture in the GOP. In general, in order to reduce the amountof code in a P picture, it should be predicted from a picture astemporally close as possible. Therefore, for the same coding bit rate, apicture can be encoded with higher image quality if it is predicted froma picture closer than the initial I picture. Access points are set atintervals such as 0.5 seconds or 1.0 second, but when a P picture ispredicted from a picture this temporally remote, the amount of codeddata is quite likely to increase. It can therefore be anticipated thatimage quality will be inferior to that obtained when no access pointsare set in the interiors of GOPs.

In the description of the first embodiment, bits were read from the discat a rate comparatively close to the coding bit rate. When reading isperformed at a considerably higher rate than the coding rate, the datain the interval preceding the intended access point can be read within arelatively short time without a seek operation. Therefore, if thefollowing encoding constraints are imposed, access points can bepredicted from P pictures relatively close by, and image quality will beimproved:

-   1. A picture used as an access point must be an I picture or a P    picture;-   2. A P picture used as an access point must be predicted from the    initial I picture of the GOP, or a P picture temporally preceding    the access point, and the pictures necessary for decoding a P    picture used as an access point must not include any B pictures;-   3. Pictures following a P picture used as an access point must not    be predicted or referenced from pictures preceding the access point,    other than the initial I picture of the GOP or a P picture used as    an access point.

The difference from the first embodiment is that when a P picture usedas an access point is encoded it may be predicted from any P picturedisposed between the I picture and the access point, provided theprevious pictures necessary for decoding the P picture at the accesspoint do not include any B pictures. FIG. 8 shows the structure of a GOPencoded under the above constraints. In FIG. 8, P5, which is an accesspoint, is predicted from P4, P4 from P3, P3 from P2, and P2 from I1.Owing to the above encoding constraints, to decode access point PS, onlyfour other pictures (I1, P2, P3, and P4) have to be decoded. P1 in FIG.8 is not a picture necessary for decoding access point P5, so P1 couldbe predicted from a B picture.

As described in the first embodiment, when no encoding constraints areimposed, eighteen pictures preceding P5 may have to be decoded in orderto decode P5. The third embodiment can significantly decrease the numberof pictures to be decoded.

It will assumed here that the reading rate from the disc is 10 Mbps, thecoding rate is 2 Mbps, the coding ratio of I pictures to B pictures to Ppictures is 10:6:1, and the time necessary for decoding a P picture is20 msec. The amount of data from I1 to P5 is about 1.5 Mbits, whichtakes about 150 msec to read. Decoding up to P5 takes about 100 msec. Inthis case, as more time is necessary for reading than decoding, P5 canbe decoded in 150 msec. In this manner, when the coding rate isconsiderably lower than the reading rate, access points can bereproduced quickly if the above constraints are imposed when the dataare encoded. This would be impractical if the coding rate were 10 Mbps,as in the first embodiment, because then reading the data would takeabout 750 msec, which is five times 150 msec.

Next, a method of selecting the P pictures necessary for decoding accesspoint P5 from the continuously read sequence of picture data will bedescribed with reference to FIGS. 9, 10 and 11. FIG. 9 shows thestructure of the reproducing device 120, and FIG. 10 shows the structureof GOP_access_info, which gives information about access within eachGOP. The reproducing device 120 in FIG. 9 differs from the reproducingdevice 100 in FIG. 4 in that when data stored in the buffer memory 108are output to the decoder 107, the output pictures are selected by apicture selector 110 that selects pictures necessary for decoding accesspoints based on information indicating whether the pictures arenecessary or not.

As the pictures are automatically selected in the LSI chip, GOP_accessinfo precedes each I picture access point in the video file 3 in FIG. 5.In the case of MPEG4-AVC, GOP_access_info is commonly recorded in a userregion in the SEI (Supplemental Enhancement Information). SEI is amanagement information region dispersed in the video file, preceding theI picture at each access point.

In FIG. 10, ref_IAP_id gives a number for identifying an I pictureaccess point in, for example, the Entry_map( ) shown in FIG. 3. However,ref_IAP_id is not particularly necessary information. The next item,number_of_P_picture_in_GOP, gives the total number of P pictures presentfrom the I picture at the access point identified by ref_IAP_id up tothe last access point in the GOP. The following for loop statement is aloop repeated this (number_of_P_picture_in_GOP) number of times.frame_num gives information from which the presentation time of a Ppicture is calculated as described in the second embodiment. Althoughframe_num is used for the calculation here, PTS can be used instead, asin the first embodiment. P_SCN_AP gives information indicating thesector offset of the intended picture relative to the start of the GOP.Use of P_SCN_AP enables the location of the P picture data for theintended access point to be calculated directly and accessed rapidly.

picture_type gives attribute information that indicates if each Ppicture is an access point, or is necessary for decoding the next Ppicture access point. picture_type has the value ‘2’ when the P pictureis an access point itself, the value ‘1’ when the P picture is a Ppicture necessary for decoding the next access point, and the value ‘0’otherwise. From the above information, the presentation time andposition of a P picture used as an access point in a GOP and thepresentation time and position of the P pictures necessary for decodingthe access point can be obtained.

In FIG. 11, ref_IAP_id gives a number for identifying an I picture,which is an access point, in, for example, the Entry_map( ) shown inFIG. 3. The number_of_PAP item gives the total number of P pictureaccess points from the I picture access point identified by ref_IAP_idto the next I picture access point. The following for loop statement isa loop repeated this (number_of_PAP) number of times. P_PTS_AP[PAP_id]and P_SCN_AP[PAP_id] give information specifying the presentation timeand position of the P picture identified by the identification number[PAP_id], which identifies a P picture used as an access point, asdescribed in the first embodiment. P_PTS_AP is necessary to identify theP picture access point closest to the time specified by the user in atime search, but the P_SCN_AP information is not particularly necessary.P_SCN_AP can, however, be used instead of or together with P_PTS_AP.number_of_P_picture gives the total number of P pictures present betweentwo access points. The following for loop statement is a loop repeatedthis (number_of_P_picture) number of times. picture type gives attributeinformation indicating if each P picture is necessary for decoding thenext P picture access point, taking, for example, the value ‘1’ when theP picture is necessary and the value ‘0’ when it is not. The appended[PAP_id] [P_id] means that the picture may be identified by either its Ppicture access point identifier [PAP_id] or its P picture identifier[P_id].

In FIG. 9, the data read out from the address given in I_SCN_AP inEntry_map( ) are temporarily stored in the buffer memory 108, then sentto the picture selector 110. The picture selector 110 selects thepictures necessary for decoding access point P5 according to thepicture_type information and transfers them to the decoder 107. As thepicture boundaries can generally be identified by header informationpresent at the start of each picture, positional information does nothave to be added for each picture, but if there is no such headerinformation, positional information for each picture may be added inGOP_access_info.

The same effect can be obtained in this embodiment if the pictureselector 110 precedes the buffer memory 108 instead of the decoder 107,and selects the picture data necessary for decoding before storing thedata in the buffer memory 108.

Next, the operation in a time search in which the user specifies apresentation time will be described. FIG. 12 shows the Entry_map( ) inthe third embodiment. The differences from the first embodiment are thatno information follows number_of_PAP, and that SEI_SCN_AP[IAP_id]replaces I_SCN_AP[IAP_id]. SEI_SCN_AP[IAP_id] gives positionalinformation about the SEI that immediately precedes the I picture at theaccess point identified by the number [IAP_id], specifying the positionof the SEI in the video file or on the disc. In this embodiment, thesector offset of the SEI immediately preceding the I picture accesspoint is given relative to the start of the video file. A byte offsetmay be used instead of a sector offset: any information may be used thatcan identify the position of the SEI relative to the start of the videofile, or the absolute position of the SEI on the disc.

In the third embodiment, there is no need for a direct seek to a Ppicture access point in the GOP, because the data are read from the Ipicture access point to the P picture access point without a seekoperation. Therefore, only information about I picture access points isnecessary in Entry_map( ). In order to read an I picture at an accesspoint, its SEI, which gives management information from the I pictureaccess point to the next I picture access point, must be read first, sothe SEI_SCN_AP information that gives the position of the SEI isrecorded in Entry_map( ).

When the user specifies a time or an image to be reproduced, Entry_map(), which is a management region separate from the content data region,is accessed, and the I picture access point closest to the specifiedtime is identified as in the first embodiment. Next, the GOP_access_infoidentified by the ref_IAP_id number corresponding to this I pictureaccess point is accessed, based on the access point information of theidentified I picture, particularly on the SEI_SCN_AP information givingthe position of the SEI.

A difference from the first embodiment is that data are now readcontinuously, starting from the address specified by I_PTS_AP(k) for theidentified I picture access point. From among the data read precedingthe P picture access point closest to the intended time, the P picturesnecessary for decoding the access point are decoded; then the accesspoint data and all of the data read thereafter are decoded to performreproduction starting from the P picture access point closest to theintended time.

FIGS. 10 and 11 illustrate the case in which GOP_access_info( ) ispresent in the video file, but a similar function can be realized ifthis information is given in Entry_map( ) in the navigation information.Note that as the navigation information is commonly interpreted by theCPU 102, if GOP_access_info( ) is given in Entry_map( ), pictureselection information needs to be sent from the CPU 102 to the pictureselector 110 every time the start of a GOP, for example, is accessed.

As described above, when the bit rate is considerably higher in readingthan in encoding, access points can be specified at appropriateintervals without reducing the number of the access points, even in alow-bit-rate encoding scheme such as MPEG4-AVC, by using I pictures andP pictures as access point pictures designated as randomly accessiblepositions.

When a picture used as an access point is a P picture, coding efficiencyand high image quality can both be achieved by coding the picture byprediction either from a temporally preceding P picture or from theinitial I picture in the GOP.

Furthermore, reproduction following the access point can proceedsmoothly because pictures temporally following an I picture or a Ppicture used as an access point are not coded by prediction frompictures temporally preceding the I picture or the P picture used as theaccess point, other than the initial I picture in the GOP and otherpictures used as access points.

Storing information on the optical disc or other video informationrecording medium giving the presentation time of pictures used as accesspoints enables quick reproduction from access points in the interiors ofGOPs. Furthermore, whether a picture is necessary for decoding the nextaccess point or not can be determined easily because attributeinformation indicating if the picture is necessary or not is recorded.

According to the present embodiment, compression efficiency can beimproved by increasing the GOP length without compromising image qualityor random accessibility.

Fourth Embodiment

In the case described in the third embodiment, the coding rate was lowerthan the reading rate, but when the coding rate does not differsignificantly from the reading rate, the data reading time becomesimpractically long. Suppose that the coding bit rate is 8 Mbps, and thecoding ratio of I pictures to B pictures to P pictures is 4:2:1. Theamount of data preceding access point P5 in FIG. 8 will then be about 5Mbits, which takes 500 msec to read; this is considerably longer thanthe time necessary for reading when low-bit-rate encoding is performed.To resolve this problem, it is necessary to read only the data neededfor decoding the P picture access point and omit other data. Forexample, removing the data not needed for decoding P5 from the datapreceding the access point at P5 in FIG. 8 can reduce the reading timeby half by reducing the amount of data to about 2.5 Mbits. Reading ofonly the data needed for decoding an access point can be realized byencoding under the following constraints:

-   1. A picture used as an access point must be an I or P picture;-   2. A P picture used as an access point must always be predicted from    the initial I picture of the GOP or a P picture preceding the access    point, and the pictures necessary for decoding a P picture used as    an access point must not include any B pictures;-   3. Pictures following a P picture used as an access point must not    be predicted or referenced from pictures preceding the access point,    other than the initial I picture of the GOP or a P picture used as    an access point; and-   4. The P pictures used for predictively encoding a P picture used as    an access point, that is, the P pictures necessary for decoding,    must be arranged in continuous succession immediately following an I    picture or a P picture used as an access point.

The fourth embodiment differs from the first embodiment in that thefourth constraint is added, and that in the second constraint, an accesspoint can be predicted from a P picture not used as an access point. Inthe first embodiment, predictions were made from an I or P pictureaccess point in order to avoid time-consuming seek operations. In thefourth embodiment, however, because the P pictures necessary fordecoding an access point are arranged in a continuous series,predictions can be made from closer P pictures, and the amount of codeat a P picture access point can be reduced.

In MPEG4-AVC, although pictures must be input to the decoder in theorder in which they should be decoded, the pictures may be arrangedarbitrarily when recorded. This embodiment resolves this inconsistencyby proposing a new GOP structure. FIG. 13 shows the relationship betweena sequence of pictures arranged in their presentation order(reproduction order, upper sequence), and the sequence is reordered atthe time of recording for random access reproduction (lower sequence).

In FIG. 13, P4 at the position of access point AP is encoded byprediction from P3, P3 is encoded by prediction from P1, and P1 isencoded by prediction from I1. Therefore, since I1, P1, and P3 have tobe decoded in this order to decode P4, P1, and P3 follow I1. P2, whichis not necessary for decoding P4, need not be included in this sequence.Pictures following P3 are arranged in the order in which they aredecoded. That is, the P pictures necessary for decoding P4 at accesspoint AP have been extracted from the sequence of pictures arranged inthe decoding order, the extracted P pictures have been placedimmediately after the I picture, and the gaps in the picture sequenceleft by the extraction of the extracted P pictures have been closed up.

When, as above, the pictures are not arranged in their decoding order,they must be reordered in the decoding order before being sent to thedecoder. FIG. 14 shows the configuration of a GOP_structure( ) whichgives information necessary for reordering. GOP_structure( ) forms apart of the navigation data 1 (FIG. 5). This embodiment will bedescribed on the assumption that there is one GOP_structure( ) permotion video file. number_of_GOP gives the total number of GOPs in themotion video file. The following for loop statement is a loop repeatedthis (number_of_GOP) number of times. GOP_PTS[GOP_id] is a presentationtime that gives the reproduction timing of the initial picture of theGOP. [GOP_id] is a number specifying the specific GOP to which the datapertains; the meaning of this notation is the same below, so subsequentexplanations will be omitted. number_of_picture gives the total numberof pictures in the GOP. The next for loop statement is a loop repeatedthis (number_of_picture) number of times. decode_order[GOP id][picture_id] gives the order in which the pictures in the GOP aredecoded. [GOP_id] [picture_id] indicates that the data pertain to[picture_id], which specifies a particular picture in the GOP identifiedby [GOP_id]; the meaning of this notation is the same below, sosubsequent explanations will be omitted. The CPU 102 rearranges the readdata in the buffer memory 108, based on the picture decoding orderinformation, and transfers the reordered pictures to the decoder 107 inthe decoding order. This embodiment is described on the assumption thatGOP_structure( ) is located in the navigation data 1 in FIG. 5, butGOP_structure( ) does not necessarily have to be treated as navigationdata 1. For example, GOP_structure( ) may be stored in a region forrecording control information specially reserved at the start of eachGOP (preceding the video data) or, in the case of MPEG4-AVC, in userregions in the SEI (Supplemental Enhancement Information); either schemehas the same effect. In these schemes number_of_GOP is not necessary; itis only necessary to give the information given in the number_of_GOPloop.

Next, the operation in a time search in which the user specifies apresentation time will be described. FIG. 15 shows the relationshipbetween Entry_map( ) and the sequence of pictures. When the userspecifies a desired time or a desired image to be reproduced, datareading starts from the address identified by I_PTS_AP(k) which isclosest to the specified time, as in the first embodiment. The amount ofdata read is the number of sectors given in Size_of_IAP, but whereas inthe first embodiment this was the total number of the sectors in the Ipicture, in the fourth embodiment Size_of_IAP gives the data size of thepictures necessary for decoding the next access point AP after the Ipicture access point, that is, the data size of the pictures necessaryfor decoding P4. In FIG. 15, this is the total number of sectors in I1plus the total number of following sectors in P1, P2, and P3. Similarly,Size_of_PAP gives the data size of the pictures necessary for decodingthe next access point (not shown) after P4, the picture located ataccess point AP: in this case, the total number of sectors in P4, P5,P6, and P7. The operation of this embodiment is similar to the operationof the first embodiment, so further description will be omitted.

As described above, using I pictures and P pictures as access pointsdesignated as randomly accessible positions enables access points to bedesignated at reasonable intervals, without reducing the number ofaccess points, even in low-bit-rate coding systems such as MPEG4-AVC.

When a picture used as an access point is a P picture, coding efficiencyand high image quality can both be achieved by coding the picture byprediction either from a temporally preceding P picture or from theinitial I picture in the GOP.

Furthermore, reproduction following the access point can proceedsmoothly because pictures temporally following an I picture or a Ppicture used as an access point are not coded by prediction frompictures temporally preceding the I picture or the P picture used as theaccess point, other than the initial I picture in the GOP and otherpictures used as access points.

When a P picture in the interior of a GOP is used as an access point,re-arranging the sequence of pictures so that the I picture and anyother P pictures used for predictively encoding the P picture, that is,the I and P pictures necessary for decoding the P picture, are bunchedtogether can shorten the access time to the access point.

Storing information on the optical disc or other video informationrecording medium giving the presentation time, position, and data sizeof pictures used as access points enables quick reproduction from accesspoints in the interiors of GOPs.

Furthermore, re-arrangement of pictures at the time of decoding isfacilitated by the recording of the decoding order of the pictures foreach GOP (video unit).

According to the present embodiment, compression efficiency can beimproved by increasing the GOP length without compromising image qualityor random accessibility.

Fifth Embodiment

Increasing the GOP length has an effect not only on random accessreproduction as described above; it also has considerable effect ontrick reproduction modes such as fast-forward reproduction. When the GOPlength is increased in the MPEG4-AVC scheme, the interval between Ipictures extends to a few seconds, which adversely affects video qualityin a fast-forward reproduction mode in which, for example, only Ipictures are reproduced. In this embodiment, a method of fast-forwardreproduction with high quality, obtained by adding improvements to thefourth embodiment, will be described. In the following explanation,descriptions of elements similar to elements in the fourth embodimentwill be omitted.

In general, in order for fast-forward reproduction to proceed smoothly,not only I pictures but also P pictures in the interiors of GOPs must bereproduced. In DVD, for example, smooth fast-forward reproduction isrealized by use of positional information about the first three Ppictures in each GOP, which is given in the navigation data. In DVD, oneGOP generally lasts 0.5 second and has about four P pictures. Therefore,if the given positional information is used to reproduce the first threeP pictures in a GOP, that covers almost all of the P pictures in theGOP, making smooth fast-forward reproduction possible. When the lengthof the GOP is increased to a few seconds or more, however, the number ofP pictures in the GOP increases, and smooth reproduction becomesdifficult if reproduction is based only on the positional informationabout the first three pictures. Also, even if positional information forall P pictures is known, reading all the P pictures is impractical,because of the low seek speed of an optical disc. In the fourthembodiment, in order to achieve random accessibility, an access pointpicture is immediately followed by the P pictures necessary for decodingthe next access point. The P pictures necessary for decoding accesspoints are not exactly the same as the P pictures necessary forfast-forward reproduction, so smooth fast-forward reproduction cannot beobtained from this arrangement alone, but if the P pictures necessaryfor fast-forward reproduction are incorporated into in this arrangement,the P pictures in the GOP can be read without repeated seek operations.

In this embodiment, the constraints imposed on the encoding forrealizing both random accessibility and fast-forward reproduction are asfollows:

-   1. A picture used as an access point or in fast-forward reproduction    must be an I picture or P picture;-   2. A P picture used as an access point or in fast-forward    reproduction must always be predicted from the initial I picture of    the GOP or another P picture, and the pictures necessary for    decoding a P picture used as an access point must not include any B    pictures;-   3. Pictures following a P picture used as an access point must not    be predicted or referenced from pictures preceding the access point,    other than the initial I picture of the GOP or a P picture used as    an access point; and-   4. The P pictures necessary for decoding access points and the    pictures displayed in fast-forward reproduction must be arranged in    continuous succession immediately following an I picture or a P    picture used as an access point.

A method of arranging and reproducing data for high quality fast-forwardreproduction, obtained by adding improvements to the fourth embodiment,will be described with reference to FIG. 16. FIG. 16 shows therelationship between a sequence of pictures arranged in their displayorder (upper sequence), and the picture sequence re-arranged for randomaccess and fast-forward reproduction at the time of recording, (lowersequence). It will be assumed that the P pictures necessary for decodingP5 at access point AP are P1 and P3, and the P pictures to be decoded infast-forward reproduction are P1 and P4. In this case, P1, P3, and P4follow I1 in continuous succession as shown in the lower sequence. Ppictures such as P2, which are used for neither purpose, are notincluded in this sequence. I1, P1, and P3 are decoded for access toaccess point AP, while I1, P1, and P4 are decoded in fast-forwardreproduction. Since the P pictures to be decoded must be selectedaccording to the purpose, the Entry_map( ) structure described in theabove embodiments must be modified. FIG. 17 shows an index informationstructure modified for random access and fast-forward reproduction.

The italics in FIG. 17 indicate parts that differ from Entry_map( ) inthe fourth embodiment; the following description will be limited tothese parts. In FIG. 17, number_of_P_picture gives the total number of Ppictures necessary for random access and fast-forward reproduction. Thefollowing for loop statement is a loop repeated this(number_of_P_picture) number of times. attribute is a flag givingpicture attribute information by indicating whether the picture is usedin random access and/or fast-forward reproduction, by taking, forexample, the value ‘01’ when the picture is used in trick reproduction,the value ‘10’ when the picture is used in random access, and the value‘11’ when the picture is used in both modes. For example, P1, P3, and P4in FIG. 16 have attribute values of ‘11’, ‘10’, and ‘01’, respectively.The addition of this type of information indicating whether a picture isnecessary in trick reproduction or random access makes it easy toidentify the P pictures necessary for random access or fast-forwardproduction. In FIG. 17, the notation [IAP_id] [P_id] or [IAP_id][PAP_id] [P_id] following attribute indicates that the attributepertains to the P picture identified by [P_id] in the interval from theI picture access point identified by [IAP_id] to next I picture accesspoint, or to the P picture identified by [P_id] in the interval from theP picture access point identified by [PAP_id] to the next P pictureaccess point within the range from the I picture access point identifiedby [IAP_id] to the next I picture access point.

In the fourth embodiment Size_of_IAP and Size_of_PAP gave the totalnumber of sectors necessary for random access, that is, the data size ofthe pictures necessary for decoding the next access point after a givenaccess point, but in the present embodiment, Size_of_IAP and Size_of_PAPgive the total data size (total number of sectors) of the picturesnecessary for decoding the next access point after the current accesspoint and the pictures necessary for fast-forward reproduction.

When a user specifies fast-forward reproduction, the transition tofast-forward reproduction occurs at the start of the next GOP. Theoperation through the reading of the amount of data specified bySize_of_IAP is exactly the same as in random access. The pictures thusread are all decoded, in order to decode from the next access point, butthe pictures actually reproduced are only those with attribute flagsspecifying fast-forward reproduction. When the reading of the amount ofdata specified by Size_of_IAP is completed, a seek operation isperformed to read the amount of data specified by Size_of_PAP from thenext P picture access point. Similar operations are repeated until aninstruction to stop fast-forward reproduction is received from the user.Smooth fast-forward reproduction is realized by the above operations.The operations during random access are exactly the same except that,based on the attribute values, the pictures necessary for decoding thenext access point are selected from among the pictures read according tothe data size specified by Size_of_IAP or Size_of_PAP; a descriptionwill be omitted. Incidentally, although the preceding descriptionassumes that the decoder 107 in FIG. 4 selects pictures from among thedecoded pictures according to the value of attribute, the pictures maybe selected by the picture selector 110 shown in the third embodimentbefore being sent to the decoder 107.

A method of improving image quality in fast-forward reproduction byadding further improvements to the fourth embodiment has been describedin the present embodiment, but fast-forward reproduction can also berealized in the third embodiment, in which the pictures do not have tobe reordered, by adding an attribute flag indicating whether or not apicture is used in random access and/or fast-forward reproduction, inplace of the picture_type item shown in the third embodiment.

As described above, using I pictures and P pictures as access pointsdesignated as randomly accessible positions and as pictures necessaryfor trick reproduction such as fast-forward reproduction enables accesspoints to be designated at reasonable intervals and makes smooth trickreproduction possible, even in low-bit-rate coding systems such asMPEG4-AVC.

When a picture used as an access point or a picture necessary for trickreproduction is a P picture, coding efficiency and high image qualitycan both be achieved by coding the picture by prediction either from atemporally preceding P picture or from the initial I picture in the GOP.

Furthermore, reproduction following the access point can proceedsmoothly because pictures temporally following an I picture or a Ppicture used as an access point are not coded by prediction frompictures temporally preceding the I picture or the P picture used as theaccess point, other than the initial I picture in the GOP and otherpictures used as access points.

When a P picture in the interior of a GOP is used as an access point oris necessary for trick reproduction, or both, re-arranging the sequenceof pictures so that the I picture and any other P pictures used forpredictively encoding the P picture, that is, the I and P picturesnecessary for decoding the P picture, are bunched together can shortenthe access time to the access point.

Storing information on the optical disc or other video informationrecording medium giving the presentation time, position, and data sizeof pictures used as access points enables quick reproduction from accesspoints in the interiors of GOPs.

Recording attribute information indicating whether a picture is apicture necessary for decoding of the next access point, a picturenecessary for trick reproduction such as fast-forward reproduction, or apicture necessary for both of these purposes permits easy determinationof whether a picture is a P picture used in random access or trickreproduction, and facilitates reordering when pictures are reorderedduring decoding.

As shown in the fourth embodiment, reordering of pictures at the time ofdecoding is facilitated by the recording of the decoding order of thepictures for each GOP (video unit).

According to the present embodiment, compression efficiency can beimproved by increasing the GOP length without compromising imagequality, random accessibility, or smooth trick reproduction.

1. A playback method for reproducing video data recorded on a medium,said video data comprising a plurality of video units which each includean intra coded I-picture, a predictive coded P-picture including a groupof blocks predicted from one picture and a bidirectionally-predictivecoded B-picture including a group of blocks predicted from two pictures,at least one of said video units including an access point P-picturecoded by motion compensation prediction using an I-picture located atthe beginning of a video unit or a selected preceding P-picture, whereina P-picture or a B-picture following said access point P-picture iscoded without referring to any picture located before said access pointP-picture, said medium containing an entry point map used for accessingsaid access point P-picture, said entry point map including a list ofdata for identifying a location of video information units designated asaccess units on said medium; said method comprising steps of:identifying an access unit which includes a start picture to display, inthe case of starting playback from said start picture; reading saidentry point map to identify a location of said access unit whichincludes said start picture; accessing said access unit to decode aclosest access point P-picture preceding said start picture; anddecoding said start picture using said access point P-picture as areference picture, wherein said start picture is decoded withoutreferring to any picture located before said access point P-picture. 2.A playback apparatus for reproducing video data recorded on a medium,said video data comprising a plurality of video units which each includean intra coded I-picture, a predictive coded P-picture including a groupof blocks predicted from one picture and a bidirectionally-predictivecoded B-picture including a group of blocks predicted from two pictures,at least one of said video units including an access point P-picturecoded by motion compensation prediction using an I-picture located atthe beginning of a video unit or a selected preceding P-picture, whereina P-picture or a B-picture following said access point P-picture iscoded without referring to any picture located before said access pointP-picture, said medium containing an entry point map used for accessingsaid access point P-picture, said entry point map including a list ofdata for identifying a location of video information units designated asaccess units on said medium; said playback apparatus comprising: meansfor identifying an access unit which includes a start picture todisplay, in the case of starting playback from said start picture; meansfor reading said entry point map to identify a location of said accessunit which includes said start picture; means for accessing said accessunit to decode a closest access point P-picture preceding said startpicture; and means for decoding said start picture using said accesspoint P-picture as a reference picture, wherein said start picture isdecoded without referring to any picture located before said accesspoint P-picture.